1. Field of the Invention
The present invention relates to a method for growing a nitride semiconductor layer on a sapphire substrate.
2. Description of the Related Art
Nitride semiconductors have been studied, developed, and put into practical use as a material for light emitting diodes (LEDs) to emit high-brightness ultraviolet light, blue light, green light and the like, and for high electron mobility transistors (HEMTs) used for high power application or the like.
In growing a crystal of the nitride semiconductor, a single-crystal substrate formed of sapphire or silicon carbide as a hetero-substrate is mainly used. Especially, the sapphire substrate is widely used because it is stable at ordinary crystal growth temperature (about 1000° C.) of the nitride semiconductor and can easily provide for a substrate with a large diameter at low cost.
As a method for growing the nitride semiconductor on the sapphire substrate, “two step growth method” using a metal-organic vapor phase epitaxy (MOVPE) method is known (See JP-B-H08-8217).
FIGS. 5A to 5D show a crystal growth method based on the two step growth method, where FIG. 5A shows a hydrogen cleaning step, FIG. 5B shows a growth step for a low temperature growth buffer layer, FIG. 5D shows a growth step for a single-crystal nucleus, and FIG. 5D shows a growth step for a GaN layer. Furthermore, FIG. 6 is a diagram for showing the temperature sequence in the two step growth method, wherein (a) through (d) correspond to the respective steps in FIGS. 5A through FIG. 5D.
The two step growth method is conducted by the following steps:
(a) The surface of a sapphire substrate 101 is sprayed with hydrogen gas 102 at about 1200° C. to clean the substrate surface (hydrogen cleaning).
(b) A low temperature growth buffer layer 103 formed of GaN, AlN or the like is grown on the sapphire substrate 101 in atmosphere of 500 to 600° C. In this step, since the crystal growth temperature is lower than the melting point of GaN, AlN or the like, the low temperature growth buffer layer 103 is grown in polycrystalline form.
(c) The sapphire substrate 101 and the low temperature growth buffer layer 103 are heated (annealed) to about 1100° C., whereby single-crystal nuclei 104 are partially formed in the low temperature growth buffer layer 103.
(d) In atmosphere of about 1100° C., a GaN layer (epitaxial layer) 105 is grown on the low temperature growth buffer layer 103 and the single-crystal nuclei 104 which function as a crystal nucleus.
In a conventional growth method prior to the two step growth method, a nitride semiconductor is directly grown on the sapphire substrate at high temperature of 1000° C. or more. However, it is difficult to obtain a continuous nitride semiconductor film which covers continuously the whole surface of the sapphire substrate. Even if the continuous film is obtained, the dislocation density of the nitride semiconductor layer must be as high as about 1010 to 1011 cm−2. The dislocation defect acts as a nonradiative recombination center or a scattering center to electron or hole, so that a semiconductor device fabricated with the nitride semiconductor produced by the conventional method cannot have device characteristics sufficient for practical use.
By using the two step growth method, a nitride semiconductor layer useful for a device can be first grown on the sapphire substrate. For example, the dislocation density of the nitride semiconductor layer can be reduced to 109 cm−2 or so, so that application of the nitride semiconductor to various devices can be finally realized.
However, the two step growth method has the following drawbacks.
(1) The two step growth method is low in production efficiency as compared to a conventional crystal growth method for GaAs- or InP-based semiconductor, and the production efficiency thereof is around half that of the conventional method. The conventional crystal growth method is generally conducted by such a simple temperature sequence with a small temperature change that temperature rises from room temperature to 500 to 600° C. (10 minutes), the crystal grows (60 minutes), temperature lowers to room temperature (20 minutes), and the crystal grown is taken out.
In contrast, as shown in FIG. 6, the two step growth method for a nitride semiconductor is conducted by a complicated temperature sequence with a remarkable temperature change where temperature rise/fall is repeated such that temperature rises from room temperature to 1200° C. (30 minutes), hydrogen cleaning is conducted in step (a) (10 minutes), temperature lowers to about 530° C. (40 minutes), a low temperature growth buffer layer is grown in step (b) (1 minute), temperature rises in step (c) (20 minutes), GaN is grown in step (d) (60 minutes), temperature lowers to room temperature (30 minutes), and the GaN grown is taken out. Consequently, the total crystal growth takes three or more hours to lower the production efficiency.
For instance, in case of growing a GaAs layer on a GaAs substrate by means of mass-production equipment, if the growth time is one hour, 10 minutes is required for temperature rise, one hour for the growth, and 20 minutes for temperature fall, so that 1.5 hours are totally required. In contrast, in case of growing GaN on a sapphire substrate as shown in FIG. 6, if the growth time is one hour, the total required time is 191 minutes (i.e., 3 hours and 11 minutes)=(30 minutes+10 minutes+40 minutes+1 minute+20 minutes+60 minutes+30 minutes). Thus, the crystal growth time is remarkably long as compared to 90 minutes in the conventional method, so that the production efficiency of the two step growth method becomes half or less that of the conventional method.
(2) The stability and reproducibility of the crystal growth is not good. In this regard, the inventor researches about the reproducibility in thickness and X-ray diffraction half width of the GaN layer by the two step growth method as below.
FIG. 7 shows the thickness characteristics of GaN layers in the case that GaN growth is conducted 100 times by the two step growth method under the same conditions, and FIG. 8 shows the X-ray diffraction half width characteristics of GaN layers in the case that the GaN growth is conducted 100 times by the two step growth method under the same conditions. In this research, the temperature sequence is used as shown in FIG. 6.
In order to obtain the characteristics of FIGS. 7 and 8, the GaN growth is conducted by steps as shown in FIGS. 5A to 5D under the next conditions. A low temperature growth buffer layer 103 as shown in step (b) is grown by supplying trimethyl gallium (TMG) at 382 μmol/min, ammonia at 10 slm, and carrier gases of hydrogen and nitrogen at 40 slm and 100 slm, respectively, where the low temperature growth buffer layer with a thickness of about 25 nm is grown for 80 sec. The annealing treatment in step (c) is conducted by supplying 20 slm ammonia, 30 slm hydrogen, and 50 slm nitrogen, and heating the sapphire substrate 101 up to 1100° C. Then, a GaN layer 105 as shown in step (d) is grown such that, when the substrate temperature rises to 1100° C., TMG is supplied at 846 μmol/min for 30 min to grow the GaN. After 30 min, the flow rate of TMG is reduced to zero to complete the growth. Then, the substrate temperature lowers to room temperature, and the substrate is taken out. The average thickness of the GaN layers 105 obtained in step (d) is 1.5 μm, and the average X-ray diffraction half width thereof is 329 sec.
As a result, the characteristics as shown in FIGS. 7 and 8 are obtained. As shown in FIGS. 7 and 8, it is found that both of the thickness and the X-ray diffraction half width of the GaN layers produced by the two step growth method vary remarkably (±20% or more relative to the average value) among the growth. Thus, the reducibility and the stability are not good.
The instability of the two step growth method is caused by the low temperature buffer growth in step (b) and the annealing in step (c) where a single-crystal nuclei 104 are formed which act as the growth origin of the GaN layer 105. The single-crystal nuclei 104 are formed by annealing the low temperature growth buffer layer 103. However, as the annealing process proceeds, the low temperature growth buffer layer 103 evaporates gradually. The amount of GaN evaporated in the vicinity of the growth temperature (near 1000° C.) depends exponentially upon the temperature, so that it varies significantly even in slight temperature change. Consequently, reflecting the slight difference of the substrate temperature among the growths, the amount and the density of the single-crystal nuclei 104 vary in the initial growth stage, whereby the above-mentioned instability is caused.
More specifically, at high temperature of 1000° C. or more, it is hard to deposit the GaN directly on the surface of the sapphire substrate and it starts growing at the single-crystal nuclei 104 as the growth origin. Thus, since the growth rate of the GaN layer in the initial growth stage depends upon the initial density of the single-crystal nuclei 104, variation in the density of the single-crystal nuclei 104 is developed as variation in the final film thickness shown in FIG. 7.
On the other hand, the fact that the X-ray diffraction half width varies remarkably means that the dislocation density of the GaN layer 105 varies significantly. The dislocation is generated inside the single-crystal nucleus as the growth origin or at the boundary where nuclei are combined each other during the growth. Accordingly, as the density of the single-crystal nuclei 104 increases, the dislocation density increases. Thus, when the density of the single-crystal nuclei 104 as the growth origin varies, the dislocation density of the GaN layer 105 grown on the single-crystal nuclei also varies. As a result, the X-ray diffraction half width varies as shown in FIG. 8.
As a method for overcoming such disadvantage of the two step growth method as mentioned above, there is known a method that a sapphire substrate is heated in a raw material gas containing nitrogen to form a nitrided region on the surface of the sapphire substrate, and a GaN layer is then grown on the nitrided region (See JP-B-H07-54806 and JP-A-2001-15443). Where the inventor conducts the above method, it is confirmed that the production efficiency is improved as compared to that of the two step growth method. Namely, the growth sequence thereof is simplified, as compared to that of the two step growth method, such that temperature rises from room temperature to 1000° C. (30 minutes), thermal nitridation treatment is conducted in ammonia (30 minutes), GaN is grown (60 minutes), and temperature lowers to room temperature (30 minutes). As a result, the total growth time is 2.5 hours that is shorter than 3 hours and 11 minutes by the two step growth method. Also, it is confirmed that the nitrided region on the surface of the sapphire substrate functions as a nucleus as the GaN growth origin to allow the growth of the GaN layer by this method.
However, the thickness and the X-ray diffraction half width of the resulting GaN layer by the above method (using the nitrided region) only exhibit the same poor reproducibility as that by the two step growth method. This is because the thickness of the nitrided regions formed on the surface of the sapphire by the thermal nitridation varies in the range of from 5 to 35 nm among the growths. Although the cause of the variation in thicknesses of the nitrided regions among the growths is not clear, the variation may be caused by the thermal instability of the nitride formed on the sapphire surface by the thermal nitridation.
Instead of nitriding thermally the surface of the sapphire substrate, a method is known in which the sapphire surface is nitrided with plasma such as nitrogen gas (See JP-A-H11-87253 and JP-A-2001-217193). Where the inventor conducts the above method, it is found that GaN with a flat surface can be obtained by growing GaN on the nitrided surface by a plasma CVD method (with raw materials of TMG and nitrogen).
However, the X-ray diffraction half width of the resulting GaN layer is around 1000 seconds which is significantly large compared to 329 seconds by the two step growth method. This means that the dislocation density of the GaN layer by the above method is remarkably higher than that by the two step growth method, which causes the deterioration in reproducibility and stability.
If nitrogen radical exists on the surface of GaN during the growth of the GaN layer, the nitrogen radical is combined instantly with Ga atom dissociated from TMG to deposit GaN since the nitrogen radical is very active. As a consequence, the surface diffusion of the Ga atom on the growth surface is hindered so that the GaN growth rate in lateral direction (direction parallel to the surface) becomes slow. Therefore, a continuous GaN layer covering the whole surface of the sapphire surface can be obtained only when the density of the nuclei as the growth origin formed on the surface of the sapphire substrate by the plasma nitridation is high. Consequently, when the continuous GaN layer can be obtained, the dislocation density must be increased.
In contrast, the MOVPE method used in the two step growth method does not involve any radical hindering the surface diffusion of Ga. Thus, since the lateral growth rate of GaN is high, the continuous GaN layer can be obtained even when the density of single-crystal nuclei is low. Accordingly, the resulting GaN layer can have low dislocation density.
As described above, in the conventional methods for producing a nitride semiconductor, it is very difficult to have stably the thermally-formed nitride on the sapphire substrate. Moreover, where the nitridation on the surface of the sapphire substrate is conducted by plasma and the GaN is grown on the nitrided surface by the plasma CVD method, the property of the resulting GaN crystal is remarkably inferior to that of the conventional two step growth method.
As such, it is impossible to overcome the low production efficiency and the instability in growth which are problematical in the two step growth method, while keeping the film quality of the GaN layer equal to or higher than that obtained by the two step growth method.